1. Field of the Invention
This invention relates to a process for preparing methacrylic acid by the catalytic vapor phase oxidation of methacrolein characterized mainly by the use of novel catalysts.
2. Description of the Prior Art
Many catalysts have already been proposed which are useful for the preparation of unsaturated carboxylic acids having from three to four carbon atoms by the catalytic vapor phase oxidation of the corresponding unsaturated aldehydes with molecular oxygen. Some of these are excellent for preparing acrylic acid from acrolein and have been used for the production of acrylic acid on a large scale. Various catalysts for preparing methacrylic acid from methacrolein have also been proposed. However, methacrylic acid has not been produced commercially from methacrolein by use of these catalysts because of the low yield and/or the relatively short lifetime of the catalysts. The former defect occurs because methacrolein has relatively high activity compared to acrolein. Consequently, in the oxidation condition, it is more readily subject to complete oxidation to carbon monoxide and carbon dioxide rather than to partial oxidation to the desired product, whereby the yield of the product is low and the development of an appropriate catalyst is difficult. As a result it would be highly desirable to produce a catalyst mixture for commercially preparing methacrylic acid.